The invention relates to ice detection systems and, more particularly, to ice detection systems for use in conjunction with ice removal systems.
The formation of ice on structures is, in certain applications, undesired and indeed dangerous. For example, the formation of ice upon exterior surfaces of an aircraft in flight is a well-known hazard, causing an increase in weight of the aircraft and a corresponding increase in fuel consumption. More importantly, however, the buildup of ice on wings and control surfaces of aircraft in flight can result in a decrease of lift and a degradation of the control function. Such conditions can result in extreme hazard to the aircraft. Accordingly, various methods have been devised for removal of ice from such aircraft surfaces.
Known methods of ice removal include the use of flexible pneumatic coverings, or "boots," on the leading edge surfaces of aircraft wings and control surfaces. These boots are periodically provided with pulses of air or other fluid to cause any ice which has accreted on the leading edge surface to be cracked away and removed by action of the airstream over the surface. Another method commonly used is to divert a portion of bleed air from the aircraft engine through passages of airframe structures having surfaces subject to icing, causing such surfaces to heat up and loosen any ice formed on the exterior portion of such surfaces.
Still another ice removal system is known as electro-impulse deicing (EIDI). In the EIDI system, a solenoid is mounted in proximity to the inner surface of a conductive thin-wall surface of the aircraft. High-current pulses of electrical energy are periodically supplied to the solenoid, causing the formation of eddy currents in portions of the conductive skin juxtaposed with the solenoid. Magnetic fields generated by such eddy currents interact with magnetic fields generated by the solenoid, producing a sudden electrodynamic force upon the portion of the surface containing the eddy currents, which fractures any ice which may have formed on the exterior surface of the skin, permitting its removal by the airstream.
All of these deicing systems have proved satisfactory in appropriate applications. However, none of such systems can be operated continuously. Continuous operation of pneumatic boots when icing conditions are not present will result in premature deterioration of the boot, while continuous operation of EIDI systems can produce unnecessary fatigue loading upon the surface of the skin. Bleed air systems require significant amounts of power, causing power to be diverted from the propulsion of the aircraft. This results in increased fuel consumption and in certain power settings of the engines, such as used during landing configuration, can result in such severe loading upon the engine as to result in a hazardous situation. It is therefore desirable to activate deicing systems only upon detection of ice upon critical aircraft surfaces.
Certain ice detection systems are known in the prior art. Many such systems employ a probe including a vibrating element, the resonant frequency of which is varied by the formation of ice upon the probe permitting such variation in frequency to be sensed and used to generate an ice indication signal. Such a device is shown, for example, in U.S. Pat. No. 3,541,540 to Hughes. Although such probes are useful in many applications it is possible, because of various aerodynamic differences between the structure of the probe and actual aircraft surfaces, that under certain conditions the probe may indicate the presence of ice when ice is not actually forming upon the critical surfaces of the aircraft. Moreover, the comparative rate of icing may be significantly different between the probe and such critical aircraft surfaces. Therefore, currently known ice detection systems are generally employed only in an advisory capacity to provide an indication of the existence of conditions amenable to ice formation upon critical aircraft surfaces and not as an indication of the actual presence of ice on such surfaces.
It is therefore desirable to provide a system for reliable indication of the actual presence of ice upon critical aircraft surfaces. It is further desirable to provide such a system which is compatible with existing types of deicing sytems and which results in a minimum increase in weight and complexity of aircraft sytems.